Diffusible hydrogen
The fraction of hydrogen in the solidified metal that moves freely through the crystal lattice. It accumulates at defects and can induce delayed cracking in weld seams.
Hydrogen is a classic problem when welding. It gets into the arc from moisture, rust, or dirt and then migrates into the molten weld pool. When the metal solidifies, the hydrogen can't escape and later tears the structure apart - often only after hours or days. That's called cold cracking, and it's particularly nasty because you don't see it right away.
To avoid that, the simple rule is: don't let hydrogen in in the first place. Electrodes must be dry and re-dried according to specifications. The joint area must be clean - so remove rust, paint, oil, and moisture. For critical steels, preheating helps, and afterwards you shouldn't let the weld cool down too quickly. When working with high-strength steels, better use filler materials with low hydrogen content - those are marked accordingly.